Cross Sections and Asymmetry Parameters for Formic Acid in the Vacuum-Ultraviolet Energy Range.

We present an experimental and theoretical investigation of the photon interaction with formic acid in the vacuum-ultraviolet energy range. The absolute absorption cross sections and ionization efficiencies were measured in the 11.2-21.4 and 13.5-21.4 eV ranges, respectively, using a double-ion chamber technique. Photoionization and neutral-decay cross sections were derived from these results. From the present ionization cross sections and previously reported ionic dissociative branching ratios, the partial cross sections for dissociating processes were obtained. Theoretically, the photoionization cross sections and the asymmetry parameters of the photoelectron angular distributions for ionization out of the six outermost valence orbitals (10a', 2a″, 9a', 1a″, 8a', and 7a') were obtained in the energy range from near-threshold to 35 eV. For that, the Padé approximant technique along with the single-center partial-wave expansion method was applied to solve the Lippmann-Schwinger equation in the static-exchange-polarization level of approximation. This is the first theoretical investigation concerning the determination of the asymmetry parameters and photoionization cross sections of formic acid in the vacuum-ultraviolet energy range. Comparison is made between our results and the previous ones.

Photoabsorption and Photoionization Cross Sections of Pyridine in the Vacuum-Ultraviolet Energy Range.

We have performed an experimental investigation into the interaction of vacuum-ultraviolet synchrotron radiation with pyridine molecules in the gas phase. Specifically, a double-ion chamber spectrometer was used to measure the absolute photoabsorption cross sections and the photoionization quantum yields from the ionization threshold to 21.5 eV. Moreover, photoionization and neutral-decay cross sections in absolute scale were derived from these data. In addition, the fragmentation pattern was investigated as a function of the photon energy by using a time-of-flight mass spectrometer and the photoelectron-photoion coincidence technique. Thus, the absolute partial ionization cross sections for each ionic fragment were obtained. Comparisons are made with experimental data available in the literature.